Max Planck Institute for Gravitational Physics (Hannover)

In 2002 the AEI Hannover branch was opened, as an extension of the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) in Potsdam. The AEI Hannover closely collaborates with the Institute for Gravitational Physics of the Leibniz Universität Hannover. Together they contribute to a new era of astronomy, which began with the first direct detection of gravitational waves on Earth on September 14, 2015. As part of this search, the AEI is a member of the LIGO Scientific Collaboration (LSC), which collects data from the world's most sensitive gravitational wave detectors, and operates the German-British gravitational-wave detector GEO600, located 20 kilometers south of Hannover. GEO600 could detect gravitational waves from the cosmic neighbourhood, if a nearby event were strong enough. The Institute also develops advanced measurement technologies and concepts for future gravitational-wave detectors. It leads the preparation and operation of the satellite missions LISA Pathfinder and eLISA (scheduled for launch in 2034) and is an important partner for the geodesy mission GRACE Follow-on.

The Hanover branch is a central partner in the global joint data analysis efforts of the LSC. To search the observational data for gravitational waves, the Hanover branch of AEI develops highly efficient analysis methods and implements them on supercomputers. For this purpose, it operates Atlas, the most powerful computer cluster in the world designed for gravitational-wave data analysis. Together with US partners, the AEI in Hanover also runs the distributed computing project Einstein@Home in which volunteers from all over the world participate in the data analysis with their PCs, laptops, or smartphones.

The Einstein@Home project makes it possible for anyone to search for gravitational waves
on their own PC, laptop or smartphone and thus become scientific explorer themselves.
Bruce Allen, Director at the Max Planck Institute for Gravitational Physics in Hannover,
is the founder of this citizen science project. The software is now also used to track down
pulsars in big data. Researchers from the Max Planck Institute for Radio Astronomy in Bonn
are also involved in this search.

Albert Einstein was right: gravitational waves really do exist. They were detected on September 14, 2015. This, on the other hand, would have surprised Einstein, as he believed they were too weak to ever be measured. The researchers were therefore all the more delighted - particularly those at the Max Planck Institute for Gravitational Physics, which played a major role in the discovery.

… is not at all where the researchers from the Max Planck Institute for Gravitational Physics want to be. The issue at hand is nothing less than the base of one of the pillars of our modern world view, the theory of general relativity. In 1915, Albert Einstein formulated, among other things, the theory that the accelerated movement of masses causes disturbances that move through space at the speed of light. He called these disturbances gravitational waves. The Earth, for instance, creates a bulge in space-time on its annual orbit around the Sun, emitting gravitational waves in the process. Given the enormous number of planets and binary stars, space must be utterly teeming with these waves. In most cases, however, the cosmic ripples are too weak to be detected with terrestrial detectors. Fortunately, there are far stronger tremors in the universe: the
dance or collision of neutron stars with black holes, or the explosion of a massive sun into a supernova. Such violent events are what scientists around the world are waiting for – for example out in a field in Ruthe, near Hanover. This is where GEO600 stretches out its two 600-meter-long arms. The evacuated stainless steel tubes measure 60 centimeters in diameter and are corrugated to increase their stability. They house the second-longest laser beam interferometer in
Europe. The measuring principle is based on the fact that gravitational waves alternately compress and stretch space. If they speed through GEO600, they will also change the paths of the laser beam that runs through the two perpendicularly arranged tubes. This tiny length difference on the order of 10-19 meters causes the light waves in the detector to fall out of step. A signal appears. Alarm! To date, however, there have been only test alarms. The researchers are working on continuously increasing the system’s sensitivity. When the cosmos quakes again, they want to finally capture the gravitational waves and thus open up a new window into space.

Albert Einstein postulated the existence of gravitational waves a century ago in his theory of general relativity, but these distortions in space-time have so far stubbornly resisted direct observation.

On 14 September 2015, the Advanced LIGO instruments detected gravitational waves for the first time ever. The signal came from the merger of two black holes, each with the mass of about 30 Suns, in a distance of 1.3 billion light-years to Earth. Albert Einstein had predicted the existence of these ripples in spacetime in 1916. The first hours of this discovery of the century took place at the Max Planck Institute for Gravitational Physics in Hannover in Bruce Allen's “Observational Relativity and Cosmology” division. The authors were also the first persons to see the signal.

The LISA Pathfinder satellite mission demonstrates core technologies for future gravitational-wave observatories in space like eLISA. These observatories will study low-frequency gravitational waves, which are emitted by, e. g., binary supermassive black holes or galactic binary stars. LISA Pathfinder was launched on December 3, 2015, and has commenced its science operations in March 2016. LISA Pathfinder will lead to a comprehensive model of all significant physical noise sources that can be extrapolated to the eLISA mission.

Gravitational waves are predicted by the general theory of relativity. Binary systems consisting of neutron stars and black holes generate these tiny ripples in space-time, which are expected to be directly measured by large-scale interferometric detectors. Not only the measurement method, but also the data analysis is paramount for the first discoveries, since only sensitive and efficient methods can filter the weak signals from the detector noise. Scientists at the MPI for Gravitational Physics (Albert Einstein Institute) have helped to bring the first discoveries closer to reality.

Two decades ago the construction of kilometer-scale gravitational wave detectors had started and the sensitivity has continuously been improved. Now these instruments are being upgraded to the second generation, which in a few years’ time will allow the first direct detection of gravitational waves and analysis of some astrophysical processes with gravitational waves. Routine gravitational wave astronomy though, will require the supreme sensitivity of third generation instruments, like the Einstein Telescope, an underground gravitational wave observatory, designed in a pan-European effort.

Pulsars are rapidly rotating, highly magnetized neutron stars which act as cosmic lighthouses by flashing at radio, X-ray or gamma-ray wavelengths. The search for gamma-ray-only pulsars is extremely difficult and computing-intensive. Even high-tech telescopes, like the one aboard the Fermi satellite, register only a few gamma-ray photons per day from such a pulsar. Using a more efficient analysis method, originally developed for detection of gravitational waves from these fast spinning neutron stars, a number of previously unknown gamma-ray pulsars have been discovered in the Fermi data.